1. Field of the Invention
This invention concerns a disk file storage apparatus of the type in which one or more rigid disks are provided for storage of data which is written to, or read from, the disks by transducers ("heads") while the disks spin. In particular, the invention concerns the measurement of spacing between the transducer mechanisms and the disks.
2. Description of the Prior Art
Data storage on rigid disks is covered extensively in Volume II of the reference work entitled MAGNETIC RECORDING, Mee, et al, eds., McGraw-Hill, 1988. In Chapter 2 of this work, a rigid disk file is described as "a stack of rigid disks" which are rotated at a high speed and whose surfaces are written to or read from using arm-mounted heads which are suspended and positioned by an actuator assembly over the surfaces. The heads are supported against disk surfaces by a thin cushion of air generated by rotation of the disks.
A disk file servo processor controls the radial position of heads with respect to disks so that selected circumferential tracks on a disk surface can be read or written to. A data channel is provided for each head. Data is recovered in a data channel by peak detection means, while head drive circuitry is provided to write data to a disk surface.
The amount of data which can be recorded on a disk (the "density" of data) is a primary indication of how well a disk file operates. One significant limitation on data density is the "spacing loss" which corresponds essentially to the distance between the head and the magnetic recording surface of a disk. Relatedly, the smaller the spacing between a head and a disk, the higher the potential data density. Variation of the distance during rotation of the disk will cause a corresponding variation in data density. Therefore, disk file manufacturing requires not only head suspension mechanisms which will stablely position a head close to a disk, but also disk whose surfaces are as flat and as defect-free as possible.
Therefore, one of the critical control evaluations made during manufacture of disk file components and assembly of disk file mechanisms is the measurement of head/disk spacing. U.S. Pat. No. 4,777,544 of Brown et al, commonly assigned with this application, well describes a method and a means for measurement of head/disk spacing. The '544 patent lays out a harmonic ratio fly height (HRF) technique for calculating head/disk spacing measurement by comparing readback spectral amplitude ratios obtained by reading a previously-recorded clearance measurement signal track at nominal and zero "flying heights".
The means of the '544 patent for taking the nominal and zero height measurements are analog components, which are impractical to integrate into a disk file mechanism because of cost, size, and performance considerations. Further, the '544 patent is directed essentially to measurement of an average flying height and makes no provision for obtaining the profile of a circumferential disk track in the form of a plurality of flying height measurements at discrete points along the track.